Thursday, October 28, 2010

Americas Settled by Two Groups of Early Humans, Study Says

"I don't want people to think that we are proposing any kind of transoceanic migration from Africa or Australia," said Neves, of the University of São Paolo in Brazil.

"We know that these [paleoindian] people had reached China around 20,000 years ago. The Mongoloid population that you see in [northeast] Asia today is more recent. So we don't have to think about transoceanic migrations to explain this."

Recent genetic studies of modern human populations have also suggested multiple early migrations across the Bering land bridge.

Neves and colleagues have not been able yet to extract ancient DNA from the Lagoa Santa remainsbut excavations are yielding additional ancient remains.

"We have already found at least 20 new skeletons older than 8,000 years that are not part of our paper," he said.

Still, not all scientists are convinced that the variations found in the skulls are proof of multiple migrations to the Americas.

"There is a huge amount of variation among the first Americans, more than you see among any other population outside of the Pacific," said Joseph Powell, an anthropologist at the University of New Mexico in Albuquerque.

"Much of that is genetic, and it comes from the fact, I think, that these first Americans had very small colonizing populations, and they have a great degree of genetic variation due to genetic drift."

Genetic drift describes random variations in a group's genetic makeup. Small populations are especially prone to the phenomenon, because the genes of a single individual play a proportionately larger role in successive generations.

American Theories

For decades most scientists believed that the first Americans were a group of hunters, known as the Clovis people, who entered the Americas via the Bering land bridge some 11,000 to 12,000 years ago.

"I think it has become more widely accepted in the archaeological community that people were here prior to Clovis," said Michael Waters, director of the Center for the Study of First Americans at Texas A&M University in College Station.

Sites in Siberia have shown that people lived in the harsh region on the Asian of the land bridge as early as 27,000 years ago, he added.

"People could survive in that Arctic environment and survive quite well," Waters said. "There would be nothing to stop them from heading east into present-day Alaska."

Moreover, sites like Chile's Monte Verde, where tools have been dated to 12,500 years ago, have bolstered the theory that people were in the Americas before the Clovis period.

"If you look at the time periods when people could have come over by land, it must have been very late, just before Clovis, or prior to the ice sheets that formed over North America reaching their maximum extent around 20,000 years ago," Waters said.

Yet the land bridge theory no longer holds a scientific monopoly.

Some scholars favor coastal migration theories, in which early settlers hopped along the Pacific coast in boats.

More controversial theorists won't rule out the possibility of ocean crossings from Europe or Africa.

However those first Americans arrived, the remains they left behind may be the only clues that could someday tell their story.